124 BIOLOGICAL LECTURES. 



mentation, but lost its pigment, taking on the exact pigment- 

 less coloration of the adult. Pigment cells are late in appearing 

 in Amblyopsis. When the young are two months old, pigment 

 is abundant. This pigmented condition is evidently a heredi- 

 tarily transmitted condition. It disappears with age. Primarily 

 this disappearance was probably individual. But, as in the 

 flounder, the depigmentation has also become hereditarily trans- 

 mitted, for even those individuals reared in the light lose the 

 color. 



Numerous facts and experiments show that, while pigment 

 may be and is developed in total darkness, the amount of color 

 in an individual animal depends, other things equal, directly on 

 the amount of light to which it is habitually exposed. 1 



The lower and upper surfaces of the flounder, the one pro- 

 tected and the other exposed to the light, give the most striking 

 example, and the argument is clinched here by the fact, noted 

 by Cunningham, 2 that a flounder whose lower side is for long 



1 A number of apparently contradictory observations may be noted : a. The 

 absence of pigment in pelagic animals or their larvae, which depend on their color- 

 less condition for their existence, is evidently due to causes entirely different from 

 those preventing the formation of pigment in cave animals. Natural selection 

 has, in pelagic animals, eliminated the color, b. The expanding of chromatophores, 

 when an animal is over a dark background or in the dark, and the contracting over 

 a light background, which may take place instantly or at the expiration of several 

 days, is evidently also a different question. The observations of Cunningham, 

 Agassiz, and Semper along this line are of interest, c. Fischel (A. M. Anat., vol. 

 xlvii, pp. 719-734, PI. XXXVII, 1893) nas noticed that larvae of salamanders reared 

 in water at 6-7 are dark, while others kept in water from 1 5-58 are light, d. Flem- 

 ming (A. M. Anat., vol. xlviii, pp. 369-374, 1896) found that with uniform temper- 

 ature in two vessels side by side, the one dark, the other light, the salamander 

 larvae in the dark vessel develop pigment cells rich in color granules ; the larvae in 

 the white vessels become pale, although the number and character of the pigment 

 cells is not otherwise changed. The difference is entirely due to the character of 

 the vessels, for, if the larvae are taken from the dark to the light vessel, they 

 become light-colored in a few days. e. Semper (Animal Life, p. 89) records that 

 "... in the tadpoles of our common toads and frogs the pigment is equally well 

 developed in yellow, blue, or red light, and in absolute darkness." This was to 

 be expected, for even in the young of cave animals pigment is, as a rule, well devel- 

 oped, f. Pouchet (Arch, de Physiol. et d'Anat., 1876, and Rev. Scient., vol. xiii, 

 1897) has demonstrated that change in color cells, such as are mentioned under 

 b and d, is brought about by the reflex control from the eye. The section of the 

 great sympathetic nerve puts an end to the changes of color under the influence 

 of light. 2 Philos. Trans., pp. 765-812. 1893. 



